Incinerator draft system

ABSTRACT

A draft system for an incinerator having a floor, upstanding walls therefrom, and a fuel feeding means mounted in the upper portion thereof, is presented. The system includes an underfire draft comprising a plurality of mutually spaced, symmetrically arrayed draft boxes mounted below the burner floor and supporting grates at the floor level. Each box is fed by a separately controlled source of forced air which is directed upwardly through the grate. The system also includes an overfire draft comprising a plurality of mutually spaced vents opening through the walls of the incinerator. Each vent is separately controlled and tangentially directed. The overfire system also may include a plurality of mutually spaced, tangentially directed blowers mounted in the walls of the incinerator. The vents and blowers of the overfire draft create a cyclonic circulation within the incinerator to facilitate uniform burning. At least one centrally located draft box may also be connected to a source of flammable gas for ignition of the material to be incinerated.

United States Patent 1 91 Stearns [451 May 1,1973

[ INCINERATOR DRAFT SYSTEM [75] Inventor: George H. Stearns, Kalispell, Mont.

[73] Assignee: Rolie's Machineworks, Inc.,

Kalispell, Mont.

22 Filed: July2, 1971 211 Appl.No.: 159,257

[56] References Cited UNITED STATES PATENTS 3,638,591 4/1970 Lausmann ..1 10/18 3,354,847 11/1967 Clement et a1. 10/7 3,472,184 10/1969 Cowan ..1lO/7 Primary Examinerl(enneth W. Sprague Attorney-LeBlanc & Shur 5 7 ABSTRACT A draft system for an incinerator having a floor, upstanding walls therefrom, and a fuel feeding means mounted in the upper portion thereof, is presented. The system includes an underfire draft comprising a plurality of mutually spaced, symmetrically arrayed draft boxes mounted below the burner floor and supporting grates at the floor level. Each box is fed by a separately controlled source of forced air which is directed upwardly through the grate. The system also includes an overfire draft comprising a plurality of mutually spaced vents opening through the walls of the incinerator. Each vent is separately controlled and tangentially directed. The overfire system also may include a plurality of mutually spaced, tangentially directed blowers mounted in the walls of the incinerator. The vents and blowers of the overfire draft create a cyclonic circulation within the incinerator to facilitate uniform burning. At least one centrally located draft box may also be connected to a source of flammable gas for ignition of the material to be incinerated.

18 Claims, 5 Drawing Figures Patented May 1, 1973 3,730,113

2 Sheets-Sheet 1 Patented May 1, 1973 2 Sheets-Shoot 2 INCINERATOR DRAFT SYSTEM This invention relates to forced air circulating systems, and in particular to a draft system for an incinerator installation. The system of this invention is preferred for use in incinerators employed at lumber mills to dispose of waste products. The invention includes separately controlled, upwardly directed draft inlets, uniformly spaced in the burner floor to provide a controlled underfire, draft, and may also include tangentially directed vents and blowers in the sides of the installation to create an overfire, cyclonic circulation within the incinerator.

Bark, sawdust, and other waste materials at sawmills are normally disposed of in an incinerator structure called a teepee burner. These installations have generally a truncated, conical shape similar to an Indian teepee, and typically have a floor diameter of from 40 to 60 feet. The standard burner, however, has a burner floor either 40 or 50 feet in diameter. Waste material is dumped into the burner through the top thereof, from a conveyor which may terminate 20 to 30 feet above the burner floor. The walls of the installation are normally constructed of large metal plates in rings on a supporting frame.

A single source of forced air with one or more outlets on the burner floor has normally been employed in prior devices to promote combustion of the waste materials. In addition, vents have been used in the sides of the burner to promote circular convection currents within the burner. United States Pat. No. 3,472,184 describes a typical structure having these features. Prior burner installations, however, have a common deficiency. Waste materials normally are consumed at a very high temperature only in the center of the un derfire draft outlet, and materials remote from the outlet or grate merely smolder and are incompletely consumed. Solid combustion products and ash emitted by incomplete combustion at these installations are an unacceptably high source of air pollutants.

The provision of side vents in an incinerator theoretically should facilitate more complete combustion, but without an adequate underfire draft they merely compound the problem by providing a cyclonic circulation within the burner to entrain partially consumed particles in the smoke emitted. Even if the side vents are thermostatically controlled, and a plurality of underfire draft outlets are provided, the smoke emitted may not meet minimum emission standards because air outlets in the burner floor tend to clog, restricting the number of outlets and increasing the air flow through the unrestricted outlets.

A further problem contributing to incomplete combustion results from the apparatus used to feed the incinerator. Ideally the fuel from the conveyor should accumulate in the center of the burner floor. However, when the fuel is dropped from a conveyor 20 to 30 feet above the floor of the burner, it tends to scatter and accumulate at peripheral areas remote from the underfire draft outlets.

When a plurality of draft outlets are provided in the burner floor, they are normally fed by a common source of forced air. If one or more of the draft outlets is covered by fuel the flow of air through the unplugged outlets increases, and uneven burning at undesirable high temperatures results.

If a single draft grate is provided, the fuel accumulates at the edges thereof. While efficient combustion occurs at or near the center of the grate, accumulated materials at remote portions of the burner floor smolder, char, or remain unburned.

The system of this invention solves the above described problems by providing a plurality of independently controlled sources of upwardly directed forced air in the burner floor. Draft grates are mutually spaced in symmetrical arrangement around the center of the dropping area for the fuel conveyor to provide complete, controlled combustion throughout the burner installation. In addition, by providing a separate source of forced air for each grate, if one grate becomes plugged with fuel, air is not forced through the remaining grates at an increased rate, resulting in undesirably high temperatures at these outlets.

The system further provides an overfire draft to create a cyclonic circulating system within the incinerator for complete and uniform combustion throughout the incinerator. The overfire system includes mutually spaced vents disposed in the wall of the incinerator surrounding the floor thereof, and optionally, may be aided by tangentially directed blowers having outlets extending through the walls of the incinerator. In the event natural air currents do not supply sufficient air for the vents, the blowers may be utilized to supplement the overfire draft from the vents. It should be noted that the blowers must be uniformly disposed at equal distant intervals surrounding the burner floor.

Accordingly, it is an object of this invention to provide an improved air circulating system for incinerators.

It is another object of this invention to provide a plurality of separately controlled, symmetrically arranged forced air outlets and vents opening into an incinerator to provide an upwardly directed, circulating draft throughout the incinerator for uniform and complete combustion of waste materials.

It is another object to provide an underfire system with symmetrically arrayed air outlets disposed in the floor in the burner for admitting a uniform underfire draft for controlled combustion of fuel thereon.

It is still another object to provide a plurality of grates mounted in the floor of an incinerator, each grate having a separately controlled source of forced air for admission therethrough to provide uniform combustion of fuel within an incinerator.

It is still another object to provide a draft system for incinerators combining separately controlled, forced air inlets spaced throughout the burner floor, and tangentially directed air inlets in the sides thereof for controlled complete combustion.

It is yet another object to provide a draft system for a waste incinerator adapted to completely combust wood and waste material whether in the form of sawdust, chips, or large pieces of bark or wood utilizing a series of symmetrically arranged, separately controlled draft boxes mounted below the incinerator floor, each box admitting forced air through a grate beneath the fuel disposed thereon, and a plurality of separately controlled, tangentially directed vents and blowers in the sides of the incinerator to admit air over the fuel for creating cyclonic convection currents within the incinerator.

These and other objects will become readily apparent with reference to the drawings and following description wherein:

FIG. 1 is a plan view of burner installation utilizing the draft system of this invention, the upper portion of the incinerator being removed to expose the burner floor.

FIG. 2 is an elevational view of the burner installation of FIG. 1, in partial section, with a schematic illustration of the means for feeding fuel to the incinerator.

FIG. 3 is a fragmentary view of a portion of the wall of the incinerator of FIGS. 1 and 2.

FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. 3.

FIG. 5 is a plan view of an optional array of underfire draft inlets which may be mounted in the floor of an incinerator.

With attention to the drawings, FIGS. 1 and 2 illustrate a portion of a conventional teepee burner, adapted to include the underfire and overfire forced air draft system of this invention. The incinerator is typically constructed in concentric rings 12 of metal plates 14 on a supporting frame (not shown). The structure is generally in the shape of a truncated cone and typically has a base diameter of 40, 50, or 60 feet.

Fuel enters the incinerator 10 from a conveyor 16 as shown in FIG. 2. Fuel drops from the conveyor 16 to the incinerator floor 18, normally a distance of 20 to feet. Because the orientation of the conveyor 16 in relation to incinerator 10 may vary from structure to structure, it is necessary through trial and error to define an approximate center of the drop zone 20 on floor 18. Center 20 often does not coincide with the center of floor 18.

The underfire draft system of this invention includes separately controlled inlets 22 mutually spaced in a symmetric array on the floor 18 of incinerator 10. The inlets surround the center of the drop zone 20. Inlets 22 are preferably draft boxes 24 as shown in FIG. 2, which support cast iron grates 26. The grates 26 may be of any conventional design.

The draft boxes 24 preferably have dimensions 36 inches by 25 inches by 24 inches deep. These dimensions, however, may vary as desired.

As shown in FIG. 2 the draft boxes 24 are preferably mounted below the floor 18 of incinerator 10 so that the grates 26 lie in a horizontal plane therewith. FIG. 1 illustrates a preferred array of inlets 22 when the floor 18 has a diameter of or 50 feet, and FIG. 5 illustrates an optional array for larger installations.

The preferred array of inlets 22 may be described as an outer grouping of draft boxes 24 and an inner grouping of draft boxes 24' about center 20. In FIG. 2 four draft boxes make up each grouping, and in FIG. 5 the outer grouping may contain four additional draft boxes 24 to cover the larger floor area. The vertical axis of each draft box 24 in the inner grouping is spaced an equal distance from center 20 and from the vertical axis of adjacent boxes. For example, in FIG. 2 the vertical axis of each draft box 24 in the inner grouping is preferably four feet from the center of the drop zone 20. A circle having a radius of four feet inscribed about center 20 will pass through each vertical axis of each draft box 24' at 90 intervals. It should be noted that the longitudinal axis of each grate 26 need not pass through center 20.

The grate boxes 24 in the outer grouping are merely symmetrically disposed about center 20, and the vertical axes of each draft box 24 need not be spaced an equal distance from the vertical axis of adjacent boxes 24.

In the preferred embodiment of FIG. 1 the vertical axes of each draft box 24 in the outer grouping defines the comers of a rectangle having the center of the drop zone 20 as its center. In addition, diagonal of this rectangle should coincide with the center line of conveyor 16. If conveyor 16 is aligned with the diagonal of the array of draft boxes 24 and 24, the likelihood that fuel will scatter beyond the periphery of the outer grouping will be minimized.

In the incinerator of FIG. 1, having a floor diameter of from 40 to 50 feet, the distance a is preferred to be about 13 feet, b about 14 feet, and 0 about 8 feet.

As stated, the array of draft boxes 24 and 24' in FIG. 5 represents an outer and inner grouping about the center of the drop zone 20 in a large incinerator such as a structure having a floor diameter of feet. The draft boxes in each grouping are symmetrically disposed an equal distance from the center 20 with a total of twelve draft boxes provided to cover a larger floor area.

It should be emphasized, however, that the draft boxes may be arrayed in any desired configuration over the anticipated drop area, and this invention is not intended to be limited to the specific number of draft boxes described in the preferred embodiment or to the array shown in FIGS. 1, 2, and 5. As will be obvious to one skilled in the art, a plurality of mutually spaced draft boxes may be disposed in any desired symmetrical array within the scope of this invention.

Forced air is admitted through each draft box 24 by conduits 30. Each conduit 30 connects the draft box 24 and a conventional blower 32. Admission of air into each conduit 30 may be controlled by a manually operated damper 34. Conduits 30 preferably are 6 inches by 8 inches in cross-sectional area.

As shown in FIGS. 1, 2 and 5 each blower 32 feeds four conduits 30 and four draft boxes 24. The blowers 32 are of a conventional paddle wheel design and preferably are driven by motors 36 having a 25 hp, 1800 rpm. capacity.

Forced air from the blowers 32 enters each of the four associated conduits 30 whether the grate 26 at a respective draft box 24 is covered by waste material or not. But, if desired, the combustion area within the burner 10 may be restricted by closing selected dampers 34 to block admission of forced air to the draft boxes 24 connected thereto.

The number of blowers 32 directing forced air into draft boxes 24, will depend upon the underfire draft desired. While one blower supplies four draft boxes in the preferred embodiment, it will be obvious to those skilled in the art that more blowers could be provided for a smaller number of draft boxes.

The overfire system of this invention includes tangentially directed vents 36 spaced equal distantly around the sides of incinerator l0, and as an optional feature, blowers 41 mounted in the sides of the incinerator 10.

The vents 36 preferably are twenty-four inches by twelve inches, in cross-sectional area and are horizontally mounted, one per panel 14, at about a 45 angle to the panel as shown at FIG. 3 and 4. Each vent 36 is provided with a damper 42 which may be manually operated, as desired. In a preferred embodiment wherein the burner floor has a diameter of 50 feet, nineteen vents 36 are mounted, one per panel, around the entire circumference of the structure. Each vent is disposed about 4 feet above the level of the burner floor.

The natural circulation through vents 36 may not be sufficient where, for example, the structure is sheltered or constructed adjacent a hill. In this situation a plurality of blowers 41 may also be installed in the walls of the structure as shown in FIG. 1, and tangentially directed to promote a cyclonic over-draft within the structure 10. Blower 41 may be driven by motors 44 having a ll) hp, 1800 rpm. capacity. Although two blowers 41 are shown in the preferred embodiment of FIG. 1, it will be obvious to those skilled in the art that additional units may be utilized provided they are mutually spaced at intervals around the circumference of the incinerator.

In addition to functioning as forced air inlets, draft boxes 24 may also be utilized to initially ignite the fuel to be incinerated. As shown in FIG. 1, a pair of draft boxes 24, in the inner grouping, may be connected by an underground pipe 51 to a source of flammable gas such as tank 52, remote from the incinerator installation. Tank 52 preferably contains propane under pressure. i

To begin incineration, wood, paper, or other dry, flammable material is placed on grates 26 and ignited. Valve 54 is then opened admitting gas through pipe 51 to nozzles (not shown) located immediately below grates 26. Blowers 32 and 41, and conveyor 16, are then actuated. When the waste material fed to the incinerator 10 on conveyor 16 is burning satisfactorily, valve 54 is closed and incineration proceeds until completed.

In summary, to eliminate the problem of incomplete combustion which leads to excessive emission of solids and ash at incinerators, a forced air draft system has been described. The draft system of this invention includes a plurality of mutually spaced, separately controlled draft inlets mounted in the burner floor and arrayed about the center of the fuel drop zone. Each of the draft inlets in the array communicates separately with a forced air source such as a conventional blower. The system of this invention also includes an overfire draft wherein a plurality of tangentially directed vents are mutually spaced at equal distances around the burner floor, and may include additional blowers also tangentially directed and mounted above the burner floor. Similar to the underfire draft, the air overfire inlets extending through the walls of the incinerator must be spaced equal distances from each other inlet. By providing a uniform arrangement of underfire air inlets, overfire vents, and overfire blowers, a uniform draft system throughout the interior of the incinerator is provided for complete combustion of the waste materials therein.

It will be obvious to those skilled in the art, that, if desired, the underfire system may be utilized alone, in conjunction with the overfire vents only, or in conjunction with the overfire vents and blowers. Implementation of the overfire system with the underfire system of this invention, will depend upon such factors as the type of fuel, the natural circulation available, the size of the incinerator, and the volume of waste material to be consumed. Implementation of one or more pair of blowers in the overfire system will also depend upon these factors, and the use of more than one pair of blowers in an overfire draft system is contemplated within the scope of this invention.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

I claim:

1. In a draft system for an incinerator structure having a burner floor, walls extending upwardly therefrom, and means for admitting fuel to the interior of the incinerator mounted in the upper portion thereof, the improvement comprising:

a plurality of mutually spaced, separately controlled.

underfire draft inlets mounted in the floor of said incinerator and opening upwardly therethrough to admit forced air into said incinerator, each of said inlets including a draft box mounted below the floor or said incinerator, the upper surface of each box forming a grate; each of said grates being disposed in a horizontal plane with the said burner floor, said inlets being symmetrically arranged in a first, inner grouping and a second, outer grouping surrounding a preselected center on the burner floor;

a plurality of blowers mounted externally to said incinerator structure;

a plurality of conduits, each of said conduits connecting one of said draft boxes and one of said blowers, so that each of said draft boxes separately communicates with an outlet of a blower through one of said conduits and at least one draft box in each of said inner and outer groupings communicates through said conduits with the outlet 0 one of said blowers;

damper means, including a removable restriction disposed within each of said conduits adjacent said blowers for controlling the flow of air through said conduits to each of said draft boxes.

2. The device of claim 1 wherein said fuel supply means comprises a conveyor extending through a port in an upper portion of said incinerator wall and terminating over said floor, said conveyor adapted to drop said fuel on a preselected portion of said floor, and said underfire draft inlets are disposed in inner and outer groupings surrounding the center of said preselected floor portion and spaced outwardly therefrom.

3. The device ofclaim 2 wherein the inner grouping of inlets comprises a plurality of first draft boxes spaced equal distantly from the center of said preselected floor portion and from each other, and the outer grouping comprises a plurality of second draft boxes radially disposed about the center of said preselected floor portion, each of said second boxes being spaced a greater distance from the center of said drop zone than said first draft boxes in the inner grouping.

4. The device of claim 1 wherein two blowers are provided and no more than four of said draft boxes are adapted to communicate with each of said blowers.

5. The device of claim 1 further comprising an overfire draft including a plurality of mutually spaced, tangentially and horizontally directed vents opening through the walls of said incinerator, said vents equal distantly spaced from each other, and mounted an equal distance above the burner floor.

6. The device of claim 5 further comprising at least one pair of tangentially and horizontally directed blowers mounted in the walls of said incinerator for providing a cyclonic circulation of air currents within said incinerator.

7. The device of claim 5 wherein said vents are spaced approximately four feet above said burner floor.

8. In a draft system for an incinerator structure having a burner floor, walls extending upwardly therefrom, and a fuel supply means including a conveyor extending through a port in an upper portion of said incinerator wall and terminating over said floor, said conveyor adapted to drop fuel in a preselected zone on said floor, the improvement comprising:

first and second groups of four, mutually spaced, un-

derfire draft boxes, each of said boxes mounted below the floor of said incinerator, the upper surface of each box forming a grate, each of said grates being disposed in a horizontal plane with the burner floor;

said first group of four draft boxes being spaced equal distantly from the center of the conveyor drop zone and from each other; said second group of four draft boxes being radially disposed about the center of the drop zone and spaced a greater distance therefrom than the draft boxes in the first group;

a pair of blowers mounted externally to said incinerator and separate conduits interconnecting said draft boxes and said blowers, each of said draft boxes separately communicating with the outlet of one of said blowers through one of said conduits, with two of said first and two of said second draft boxes communicating with each of said blowers; and a damper disposed within each of said conduits adjacent said blower, each of said dampers adapted to separately control the flow of air from each of said blowers to each of said draft boxes.

9. The device of claim 8 further comprising an overfire draft including a plurality of mutually spaced, tangentially and horizontally directed vents opening through the walls of said incinerator, said vents equal distantly spaced from each other, and mounted an equal distance above the burner floor.

10. The device of claim 9 further comprising at least one pair of tangentially and horizontally directed blowers mounted in the walls of said incinerator for providing a cyclonic circulation of air currents within said incinerator.

11. The device of claim 9 wherein said vents are spaced approximately four feet above said burner floor.

12. In a draft system for an incinerator structure having a burner floor, walls extending upwardly therefrom, and a fuel supply means including a conveyor extending through a port in an upper portion of said incinerator wall and terminating over said floor, said conveyor adapted to drop fuel in a preselected zone on said floor,

the improvement comprisinfg:

first and second groups 0 mutually spaced, underfire draft boxes, each of said boxes mounted below the floor of said incinerator, the upper surface of each box forming a grate, each of said grates being disposed in a horizontal plane with the burner floor;

said first group of draft boxes being spaced equal distantly from the center of the conveyor drop zone and from each other, said second group of draft boxes being radially disposed about the center of the drop zone and spaced a greater distance therefrom than the draft boxes in the first group;

a plurality of blowers mounted externally to said incinerator and separate conduits interconnecting said draft boxes and said blowers, each of said draft boxes separately communicating with the outlet of one of said blowers through one of said conduits with at least two of said first and at least two of said second draft boxes communicating with at least one of said blowers;

and a damper disposed within each of said conduits adjacent said blowers, said dampers adapted to separetely control the flow of air from each of said blowers to each of said draft boxes.

13. The device of claim 12 wherein two blowers are provided and no more than four of said boxes are adapted to communicate with the outlet of each of said blowers.

14. The device of claim 12 wherein three blowers are provided and no more than four of said draft boxes are adapted to communicate with the outlet of each of said blowers.

15. The device of claim 13 wherein said first and second groups of draft boxes each comprises four draft boxes with two of said first and two of said second draft boxes communicating with each of said blowers.

16. The device of claim 12 further comprising an overfire draft including a plurality of mutually spaced, tangentially and horizontally directed vents opening through the walls of said incinerator, said vents equal distantly spaced from each other, and mounted an equal distance above the burner floor.

17. The device of claim 16 further comprising at least one pair of tangentially and horizontally directed blowers mounted in the walls of said incinerator for providing a cyclonic circulation of air currents within said incinerator.

18. The device of claim 16 wherein said vents are spaced approximately four feet above said burner floor. 

1. In a draft system for an incinerator structure having a burner floor, walls extending upwardly therefrom, and means for admitting fuel to the interior of the incinerator mounted in the upper portion thereof, the improvement comprising: a plurality of mutually spaced, separately controlled underfire draft inlets mounted in the floor of said incinerator and opening upwardly therethrough to admit forced air into said incinerator, each of said inlets including a draft box mounted below the floor or said incinerator, the upper surface of each box forming a grate; each of said grates being disposed in a horizontal plane with the said burner floor, said inlets being symmetrically arranged in a fiRst, inner grouping and a second, outer grouping surrounding a preselected center on the burner floor; a plurality of blowers mounted externally to said incinerator structure; a plurality of conduits, each of said conduits connecting one of said draft boxes and one of said blowers, so that each of said draft boxes separately communicates with an outlet of a blower through one of said conduits and at least one draft box in each of said inner and outer groupings communicates through said conduits with the outlet of one of said blowers; damper means, including a removable restriction disposed within each of said conduits adjacent said blowers for controlling the flow of air through said conduits to each of said draft boxes.
 2. The device of claim 1 wherein said fuel supply means comprises a conveyor extending through a port in an upper portion of said incinerator wall and terminating over said floor, said conveyor adapted to drop said fuel on a preselected portion of said floor, and said underfire draft inlets are disposed in inner and outer groupings surrounding the center of said preselected floor portion and spaced outwardly therefrom.
 3. The device of claim 2 wherein the inner grouping of inlets comprises a plurality of first draft boxes spaced equal distantly from the center of said preselected floor portion and from each other, and the outer grouping comprises a plurality of second draft boxes radially disposed about the center of said preselected floor portion, each of said second boxes being spaced a greater distance from the center of said drop zone than said first draft boxes in the inner grouping.
 4. The device of claim 1 wherein two blowers are provided and no more than four of said draft boxes are adapted to communicate with each of said blowers.
 5. The device of claim 1 further comprising an overfire draft including a plurality of mutually spaced, tangentially and horizontally directed vents opening through the walls of said incinerator, said vents equal distantly spaced from each other, and mounted an equal distance above the burner floor.
 6. The device of claim 5 further comprising at least one pair of tangentially and horizontally directed blowers mounted in the walls of said incinerator for providing a cyclonic circulation of air currents within said incinerator.
 7. The device of claim 5 wherein said vents are spaced approximately four feet above said burner floor.
 8. In a draft system for an incinerator structure having a burner floor, walls extending upwardly therefrom, and a fuel supply means including a conveyor extending through a port in an upper portion of said incinerator wall and terminating over said floor, said conveyor adapted to drop fuel in a preselected zone on said floor, the improvement comprising: first and second groups of four, mutually spaced, underfire draft boxes, each of said boxes mounted below the floor of said incinerator, the upper surface of each box forming a grate, each of said grates being disposed in a horizontal plane with the burner floor; said first group of four draft boxes being spaced equal distantly from the center of the conveyor drop zone and from each other; said second group of four draft boxes being radially disposed about the center of the drop zone and spaced a greater distance therefrom than the draft boxes in the first group; a pair of blowers mounted externally to said incinerator and separate conduits interconnecting said draft boxes and said blowers, each of said draft boxes separately communicating with the outlet of one of said blowers through one of said conduits, with two of said first and two of said second draft boxes communicating with each of said blowers; and a damper disposed within each of said conduits adjacent said blower, each of said dampers adapted to separately control the flow of air from each of said blowers to each of said draft boxes.
 9. The device of claim 8 further comprising an overfire draft including a plurality of mutUally spaced, tangentially and horizontally directed vents opening through the walls of said incinerator, said vents equal distantly spaced from each other, and mounted an equal distance above the burner floor.
 10. The device of claim 9 further comprising at least one pair of tangentially and horizontally directed blowers mounted in the walls of said incinerator for providing a cyclonic circulation of air currents within said incinerator.
 11. The device of claim 9 wherein said vents are spaced approximately four feet above said burner floor.
 12. In a draft system for an incinerator structure having a burner floor, walls extending upwardly therefrom, and a fuel supply means including a conveyor extending through a port in an upper portion of said incinerator wall and terminating over said floor, said conveyor adapted to drop fuel in a preselected zone on said floor, the improvement comprising: first and second groups of mutually spaced, underfire draft boxes, each of said boxes mounted below the floor of said incinerator, the upper surface of each box forming a grate, each of said grates being disposed in a horizontal plane with the burner floor; said first group of draft boxes being spaced equal distantly from the center of the conveyor drop zone and from each other, said second group of draft boxes being radially disposed about the center of the drop zone and spaced a greater distance therefrom than the draft boxes in the first group; a plurality of blowers mounted externally to said incinerator and separate conduits interconnecting said draft boxes and said blowers, each of said draft boxes separately communicating with the outlet of one of said blowers through one of said conduits with at least two of said first and at least two of said second draft boxes communicating with at least one of said blowers; and a damper disposed within each of said conduits adjacent said blowers, said dampers adapted to separetely control the flow of air from each of said blowers to each of said draft boxes.
 13. The device of claim 12 wherein two blowers are provided and no more than four of said boxes are adapted to communicate with the outlet of each of said blowers.
 14. The device of claim 12 wherein three blowers are provided and no more than four of said draft boxes are adapted to communicate with the outlet of each of said blowers.
 15. The device of claim 13 wherein said first and second groups of draft boxes each comprises four draft boxes with two of said first and two of said second draft boxes communicating with each of said blowers.
 16. The device of claim 12 further comprising an overfire draft including a plurality of mutually spaced, tangentially and horizontally directed vents opening through the walls of said incinerator, said vents equal distantly spaced from each other, and mounted an equal distance above the burner floor.
 17. The device of claim 16 further comprising at least one pair of tangentially and horizontally directed blowers mounted in the walls of said incinerator for providing a cyclonic circulation of air currents within said incinerator.
 18. The device of claim 16 wherein said vents are spaced approximately four feet above said burner floor. 